1. Field of the Invention
The present invention generally relates to drive mechanisms and, more particularly, to a drive mechanism which drives a movable member provided in a vacuum chamber such as a process chamber used for a semiconductor manufacturing process.
2. Description of the Related Art
In the manufacturing processes of semiconductor devices, liquid display devices (LCDs) or plasma display devices, accurate movement and positioning of a work piece such as a semiconductor wafer must be achieved under the vacuum or negative pressure (partial vacuum) environment. Accordingly, a handling device handling a work piece under the vacuum or negative pressure environment must be provided with a movable unit such as a conveyor arm which can accurately move and position the work piece. Such a handling device operated under the vacuum or negative pressure requires a special drive mechanism for driving the movable unit including a special bearing which movably supports the movable unit. Conventionally, a ball bearing or a roller bearing is used as a bearing for supporting the movable unit in a vacuum chamber.
Japanese Patent Publication No. 6-49529 discloses a method using a magnetic floating mechanism to convey an object within a vacuum chamber. According to the method, a work piece such as a wafer is mounted on a conveyor arm that floats within the vacuum chamber by being supported by a magnetic force applied by electromagnets provided outside the vacuum chamber. The conveyor arm conveys the work piece within the vacuum chamber. Accordingly, the conveyor arm does not produce dust or particles within the vacuum chamber since the conveyor arm can move within the vacuum chamber in the non-contact manner. However, said patent document does not disclose the floating mechanism of the conveyor arm and the method for controlling the floatation of the conveyor arm.
Japanese Laid-Open Patent Application No. 62-88528 discloses an X-Y stage, which uses an air bearing for floatation of a movable unit on a cushion of air and a linear motor for driving the movable unit. The stage is formed on the crossing part of two orthogonal cross guides. Each of the cross guides is provided with air bearings on opposite ends and a liner motor formed along each cross guide. However, this patent document does not disclose the specific structure of the air bearings and method for controlling the floating force achieved by the air bearings. A structure of a conventional air bearing is disclosed in Japanese Laid-Open Utility Model Application No. 60-162731.
Conventionally, a ball bearing or a roller bearing is used as a bearing for supporting a movable unit. However, when a ball bearing or a roller bearing is operated in a vacuum chamber, the pressure in the vacuum chamber may be increased due to gas discharged from the lubricant provided in the bearing. Additionally, the lubricant may scatter within the vacuum chamber due to rotation of the balls or rollers in the bearing, which contaminates the environment within the vacuum chamber. Further, friction in the bearing causes a delay in a start of movement and inaccuracy in a stop position. There is a further problem in that a high-speed operation cannot be performed since a vibration due to sliding must be prevented so as to safely hold and move a work piece.
As mentioned above, the method disclosed in Japanese Patent Publication No. 6-49529 does not use a mechanical contact bearing such as a ball bearing or a roller bearing. However, since the conveyor arrangement disclosed in Japanese Patent Application No. 6-49529 does not disclose the method of flotation control of the conveyor arm, the conveyor arm may contact the inner bottom surface of the vacuum chamber.
A gas bearing such as an air bearing is popular for a movable unit operated under a normal or atmospheric pressure. However, there has been no suggestion to apply a gas bearing to a negative pressure environment. Actually, neither the above-mentioned Japanese Laid-Open Patent Application No. 62-88528 nor Japanese Laid-Open Patent Application No. 60-162731 considers the use of a gas bearing under a negative pressure environment. That is, the gas bearing disclosed in those patent documents is generally arranged for use under a normal pressure. Therefore, the gas bearing so disclosed is unusable under a negative pressure environment since the gas bearing exhausts some amount of air or gas, which could raise the pressure thus eliminating the negative pressure environment.
As mentioned above, there has been no suggestion as to a bearing that enables movement and positioning of a movable unit in a conveyor system operated under a negative pressure environment. If a ball bearing or a roller bearing is used, the above-mentioned problems will rise.
Additionally, the conventional floating system using a magnetic force does not provide a lift control of a movable unit. For example, in the method disclosed in the above-mentioned Japanese Patent Application No. 6-49529, there may be a case in which the lift of the conveyor arm fluctuates and cannot be maintained at a constant level, or the conveyor arm vibrates. Additionally, if the movable unit is lifted in excess, an appropriate drive control cannot be performed, and a high-speed motion and a quick response cannot be achieved.